1. Field of the Invention
The present invention relates to an image forming apparatus including a print head for applying ink to a surface layer of a recording medium transported thereto and a heating unit for heating this recording medium for fixing the ink applied to the surface layer by the print head onto its fixing layer.
2. Description of the Related Art
An exemplary conventional technique relating to the above field of art is disclosed in Japanese patent application xe2x80x9cKokaixe2x80x9d No: Hei. 10-297197. According to this, a metal substrate includes a coloring ground layer acting also as a rust-preventive layer, a transparent resin layer as an optical transparent resin layer formed over the coloring ground layer, the resin layer being made of acrylic resin, polyester resin, urethane resin etc., and an inkjet receiving layer formed over the resin layer and made of e.g. porous alumina. After application of a sublimating ink or pigment on the inkjet receiving layer by an inkjet printing, the sublimating pigment is heated in a heating furnace or by a hot press, whereby the sublimating pigment in the inkjet receiving layer is sublimed into the transparent resin layer. Then, the inkjet receiving layer is removed to obtain an ornamental metal body having a colored pattern fixedly formed within the transparent resin layer.
According to further art disclosed by Japanese patent application xe2x80x9cKokaixe2x80x9d No: 2001-105638, sublimating ink is transferred from an ink ribbon onto a surface of a recording sheet. In order to heat and fix the ink on the sheet, the sheet is charged into a heater box, in which the sheet is advanced and heated between a press roll and a heat roll opposed to each other with a small gap therebetween or between a heat roll and a conveyer belt disposed along a portion of the peripheral face of the heat roll, and then the sheet is discharged from the heater box immediately.
Further, in the field of textile printing, according to an exemplary technique disclosed by Japanese patent application xe2x80x9cKokaixe2x80x9d No: Hei. 08-311782, dye is applied to a textile by the inkjet printing method. Then, in order to reinforce the fixing of the dye and also to improve its color development, the textile is charged into a heater device to be heated therein. Then, the textile is discharged from the device immediately to be cooled at the normal temperature.
Still further, Japanese patent application xe2x80x9cKokaixe2x80x9d No: Hei. 10-16188 discloses an image forming apparatus. According to this, first, a primary image is formed on a thermal transfer sheet by e.g. an inkjet printer. Then, this thermal transfer sheet having the image formed thereon is laid over a recording sheet and these sheets are pressed and heated together, whereby the image (ink) formed on the thermal transfer sheet will be sublimed by the heat and transferred onto an ink fixing layer of the recording sheet, thus forming a secondary image thereon. With this, a finished printed product is obtained.
Another image forming apparatus is known from Japanese patent application xe2x80x9cKokaixe2x80x9d No: Hei. 10-230589. According to this, a laminated material layer is provided in advance on an ink fixing layer of a recording sheet. Then, an image is formed on the laminated material layer by e.g. an inkjet printer. Then, the resultant sheet is pressed and heated by heat rolls, thereby to make the laminated material layer transparent and also to fix the ink pigment on the fixing layer. With this, a finished printed product is obtained.
With these image forming apparatuses, sublimating ink is discharged against the recording medium which usually is being transported along a sub-scanning direction, so that an image is formed thereon with ink droplets (here, these will be referred to as xe2x80x9cun-sublimated print dotsxe2x80x9d). Then, during the subsequent heat fixing process, these ink droplets are heated to sublime, so that the sublimed ink pigment (referred to here as xe2x80x9csublimed print dotsxe2x80x9d) is fixed in the fixing layer of the recording medium, whereby a final printed image formed of the sublimed print dots with vivid color development is obtained.
In order to maintain good quality of printed product as a finished product regardless of the print size, it is essential that the heating process be effected on the recording medium applied with the ink at an appropriate timing and with appropriate heating pattern. In this regards, it should be noted that these types of image forming apparatuses are often installed in a printing service shop or a photography shop, so the apparatus needs to be formed as compact as possible.
In view of the above-described state of the art, with the image forming apparatus described at the onset as being its starting technique, a primary object of the present invention is to provide a compact image forming apparatus capable of effecting an appropriate heating process in a smooth manner on various kinds of recording media transported from the printing unit, including a recording media of the standard poster size to a very long recording medium having a length of over 10 meters for production of a huge commercial advertisement billboard or the like.
For accomplishing the above-noted object, according to the present invention, an image forming apparatus comprises a printing unit having a print head for applying ink to a surface layer of a recording medium transported thereto; a loop-forming unit capable of temporarily storing the recording medium discharged from the printing unit; and a heating unit for heating the recording medium fed from the loop-forming unit for fixing the ink applied to the surface layer onto its fixing layer; wherein said heating unit includes at least two heating areas which are adjusted to heating temperatures different from each other.
With the above construction, the recording medium introduced into the heating unit is heated in the at least two heating areas adjusted to different heating temperatures. Hence, the recording medium is subjected to a main heating condition in which the medium is heated at the ink fixing temperature for sublimation of the ink (generally at 150xc2x0 C. or higher) and a supplementary heating condition in which the medium is heated to a temperature lower than this ink fixing temperature. Namely, by appropriately avoiding rapid cooling or rapid heating, it becomes possible to alleviate thermal load to which the recording medium is exposed. Especially, when the heating unit is adapted for effecting a supplementary heating prior to the main heating, the recording medium is to be heated first to the lower temperature than the ink fixing temperature and then heated to the ink fixing temperature, whereby rapid increase in the temperature may be avoided when the recording medium is heated to the fixing temperature. As a result, with effective elimination of disadvantage due to rapid heating, a finished printed product having high quality may be obtained.
Further, according to one preferred embodiment of the invention, in order to create an advantageous heating condition within the heating unit, the heating unit includes a preliminary heating subunit for heating the recording medium from a normal temperature level to a ink fixing temperature level, a main heating unit for heating the recording medium at the ink fixing temperature level and a slow cooling subunit for slowly cooling the recording medium to the normal temperature level. With this construction, the recording medium which has been heated in the heating units is slowly cooled from the ink fixing temperature (generally 150xc2x0 C. or higher) to the normal temperature (room temperature) by the slow cooling subunit. As a result, it is possible to restrict occurrence of deformation such as wrinkles in the recording medium which would occur otherwise if the medium were discharged suddenly from the heating unit to the outside due to rapid cooling thereof from the ink fixing temperature to the room temperature. Consequently, there is obtained a finished printed product of high quality with minimum wrinkles.
Preferably, said each subunit includes a transport guide member for coming into face contact with the recording medium and a heater disposed to be able to transfer its heat to the transport guide member. With this construction, the recording medium can be heated with as uniform as possible temperature distribution while the medium is being transported. In particular, in the case of the preliminary heating subunit or the slow cooling subunit, by arranging the layout of the heater for providing the heat to its transport guide member in such a manner that the surface temperature of the transport guide member may be raised mildly along a transporting direction of the recording medium from the environment temperature (room temperature) to the ink fixing temperature or that the surface temperature may be lowered mildly along the same direction from the ink fixing temperature to the room temperature, the recording medium can be heated or slowly cooled in an uniform as well as continuous manner. In order to readily realize such effective heater layout, it will be advantageous to form the heater of an electric wire and dispose this electric wire so that its disposing density is gradually increased (in the case of the preliminary heating subunit) or gradually decreased (in the case of the slow cooling subunit) along the transporting direction.
According to one preferred embodiment of the present invention, the printing unit includes a first transporting mechanism for transporting the recording medium while the heating unit includes a second transporting mechanism for transporting the recording medium; and said loop-forming unit is capable of absorbing a transportation speed difference between the first transporting mechanism and the second transporting mechanism. With this construction, the recording medium discharged from the printing unit can be temporarily stored within the loop-forming unit. Therefore, it is possible to introduce the recording medium to the heating unit at an appropriate timing required for achievement of high-quality image and it is also possible to select the retention time of the recording medium within the heating unit from a variety of ranges. Moreover, the discharging speed of the recording medium from the printing unit need not be fixed. Instead, this discharging speed can be independent of the transport speed of the recording medium in the heating unit. These provide greater freedom in the design and adjustments of the printing unit as desired. To put it the other way around, the transport speed and/or retention time of the recording medium in the heating unit can be optimally set, independently of the discharging speed of the recording medium from the printing unit.
If the transport speed of the first transporting mechanism is set higher than the transport speed of the second transporting mechanism, the printing unit can effect a printing operation at a high speed even if the transport speed of the second transporting mechanism is set low in order to increase the retention time of the recording medium for obtaining better image quality. Particularly, in processing a very long recording medium, it is possible to avoid such inconvenience as interruption of the printing process in the middle of the same.
According to another preferred embodiment of the present invention, a cutter for cutting the recording medium is provided within the printing unit or between the printing unit and the loop-forming unit. With this, from the elongate recoding medium retained in the form of a roll in the printing unit, a necessary length of the medium can be cut as desired for use. Hence, this construction advantageously provides the capability of processing recording media of various lengths. As the cutter is disposed inside the printing unit or between the printing unit and the loop-forming unit, the recording medium can be charged into the heating unit via the loop-forming unit continuously during the cutting operation which requires keeping the recording medium temporarily still.
According to a further preferred embodiment of the invention, the printing unit, the loop-forming unit and the heating unit are detachably attached to each other. The loop-forming unit and the heating fixing unit may be integrated into a single recording medium processing unit which is detachably attachable to the printing unit. With this, it becomes possible to replace the unit with a different unit whose loop-forming unit allows accumulation of a loop of a different length or whose heating unit has a heating area having a different length. Further, it becomes also possible to detach the printing unit alone from the invention""s image forming apparatus and us the unit as a standard printer for printing on a recording medium of a conventional paper or the like which does not require fixing by heating.
According to a still further preferred embodiment of the present invention, the loop-forming unit includes an intermediate transporting mechanism for sending the recording medium received from the printing unit into the heating unit; and the intermediate transporting mechanism forms a first storage section and a second storage section each operable to store the recording medium while forming a loop thereof. Preferably, the first storage section and the second storage section are disposed one after another along the transporting direction of the recording medium. With this sequential arrangement of the two storage sections, it becomes possible to continuously transfer a plurality of recording media between the printing unit and the heating unit. In this, preferably, the intermediate transporting mechanism includes a pinching transport roller set and a feed roller set, the first storage section being formed between a recording medium exit of the printing unit and the pinching transport roller set, the second storage section being formed between the pinching transport roller set and the feed roller set. With this, it becomes possible to form the loops of the recording medium formed according to movement of the pinching transport roller set on the opposed sides of the pinching transport roller set. Consequently, in spite of the large storage capacity thereof, the loop-forming unit may be formed compact. Further, in the case of a relatively short recording medium which can be accommodated within the one first storage section, by transferring the entire medium to the second storage section immediately after completion of the printing process through the pinching transport roller set, the first storage medium may be emptied to be able to receive a next recording medium. As a result, the processing speed of the printing unit may be increased.
Advantageously, the pinching transport roller set can be adapted for acting as a curl correcting roller for alleviating curling tendency of the recording medium. With this, if the recording medium as transported to the pinching transport roller set has a curling tendency since the recording medium before being fed to the printing unit was kept in the form of a roll wound about a core, such curling tendency of the recording medium can be eliminated or at least reduced by the pinching transport roller set acting also as the curl correcting roller and then transported to the heating unit. Accordingly, it becomes possible to avoid occurrence of inappropriate transportation (including such phenomenon as jamming of the recording medium within the transporting mechanism which needs to be avoided) which would occur if the medium were sent to the heating unit with curling tendency remaining in the medium. To realize this construction, the pinching transport roller set may be constituted from a turn roller having a large diameter and a plurality of assist rollers disposed in spaced apart relationship along the peripheral face of the turn roller. With this, through the entire construction is compact, the construction allows the recording medium to be turned with a precise radius of curvature while the medium is retained by the construction reliably. For alleviating the curling tendency of the recording medium, it is preferred that the winding angle of the recording medium to the turn roller exceed 100 degrees.
Further and other features of the invention will become apparent upon reading the following detailed description of the preferred embodiments thereof with reference to the accompanying drawings.